Does anyone use the battery packs for the built in warmers on the Burton boots? Was thinking of picking up a pair for my wife for Christmas. She has the compatible boots (sapphire). Her feet get cold so fast and even in mild temps of teens to twenty's. Is anyone have experience with these? Is the system durable. Is the 119 price for a pair or each? Burton Thermic Battery Pack - Free Shipping! from Dogfunk.com